Electronic components are used to perform various functions, for example, to store data, to process data, and to communicate data. However, for such electronic components to function cooperatively, they need to be able to communicate among each other. Various signaling techniques have been developed to facilitate such communication. One such signaling technique is referred to as single-ended signaling. In single-ended signaling, a single wire, which may be any type of conductive path, may be used to communicate a signal by varying a parameter, such as a voltage on that wire with respect to a reference voltage, such as ground. Such a reference voltage may be used as a common reference voltage for several single-ended signals. Another type of signaling technique is referred to as differential signaling. In differential signaling, two wires, which may be any type of conductive paths, may be used to communicate a signal by varying a parameter of one of the wires with respect to a parameter of the other wire. Such signaling is referred to as differential signaling. The meaning or value of a signal communicated using a differential signaling system is typically determined by comparison of the parameters of the wires relative to each other, rather than relative to a common reference voltage.
Driver circuits used to produce the signals communicated between electronic components have generally been designed to provide either single-ended signaling or differential signaling, but have not generally been capable of selectably providing both types of signaling. Thus, single-ended driver circuits have not generally been compatible with differential signaling systems, and differential driver circuits have not generally been compatible with single-ended signaling systems.
To reduce reflections and other unwanted distortion of signals, the wires to which signals are applied are sometimes terminated with an appropriate termination impedance. An additional difficulty that relates to single-ended and differential signaling is that different types of terminations are sometimes used with single-ended signaling and differential signaling. For example, for single-ended signaling, a wire used for signaling preferably is center terminated (e.g., with one terminating element coupled from the wire to a first reference voltage and another terminating element coupled from the wire to a second reference voltage). As a counterexample, for differential signaling, each wire used for signaling preferably is terminated with a single terminating element coupled to a single reference voltage. Therefore, it is difficult to apply one termination scheme that optimizes performance for both types of signaling systems.
FIG. 1 is a drawing including schematic diagrams of single-reference terminations and a center termination in accordance with the prior art. In a first example of a single-reference termination illustrated in FIG. 1, wire 103 is coupled to terminating element 102, which is coupled to a first reference voltage 101. In a second example of a single-reference termination illustrated in FIG. 1, wire 104 is coupled to terminating element 105, which is coupled to a second reference voltage 106. In an example of a center termination illustrated in FIG. 1, wire 109 is coupled to terminating elements 108 and 110. Terminating element 108 is coupled to first reference voltage 107, while terminating element 110 is coupled to second reference voltage 111.
Drivers for single-ended signaling systems have not provided compatibility with differential signaling systems, and drivers for differential signaling systems have not provided compatibility with single-ended signaling systems, thereby increasing manufacturing costs and inventory requirements. Thus, neither the single-ended nor differential signaling techniques of the past have provided the flexibility to overcome these difficulties and limitations. Therefore, what is needed is a method and apparatus to overcome the disadvantages described above.